The present invention relates to disc drives. More particularly, the present invention relates to damping vibrations which occur in a rotary actuator in a disc drive.
A typical magnetic disc drive includes one or more magnetic discs, a transducer supported by a hydrodynamic air bearing which flies above each magnetic disc, and a drive controller for controlling the disc drive based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the magnetic discs and to store information on the magnetic discs.
An electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the transducer radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations.
Information is typically stored on the magnetic discs by providing a write signal to the transducer to encode flux reversals on the surface of the magnetic disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the electromechanical actuator so that the transducer flies above the magnetic disc, sensing flux reversals on the magnetic disc and generating a read signal based on those flux reversals. The read signal is then decoded by the drive controller to recover the data represented by the flux reversals stored on the magnetic disc, and consequently represented in the read signal provided by the transducer.
Conventionally, the electromechanical actuator includes an actuator arm assembly which is coupled to a head gimbal assembly (which includes the transducer and hydrodynamic air bearing). The actuator arm assembly is controlled to pivot or rotate about a shaft generally defining an axis of rotation to move the head gimbal assembly over the surface of the disc to a desired radial position. The actuator arm assembly typically includes a sleeve rotatably mounted about the shaft by a pair of bearings. The sleeve is coupled to an actuator arm and a voice coil which is connected to the actuator arm. A magnet, or group of magnets, is positioned relative to the voice coil such that when the disc drive controller causes current to flow through the voice coil, the fields generated by the voice coil interact with the magnetic field provided by the magnets and cause rotation of the actuator arm assembly about the shaft.
Such actuator arm assemblies are movable between two extreme positions. In the first extreme position, the actuator arm assembly is positioned to hold the hydrodynamic air bearing over the innermost radius of the magnetic disc. In the second extreme position, the actuator arm assembly is positioned to hold the hydrodynamic air bearing over the outermost radius of the disc.
The ball bearings which rotatably mount the sleeve to the shaft in the actuator arm assembly have an associated radial stiffness which is typically far less than infinity. Thus, the actuator arm assembly is vulnerable to vibrations at a resonance determined principally by the ratio of the bearing radial stiffness to dead weight of the mass supported by the bearings. In disc drives, the dead weight of the actuator arm assembly is typically made as low possible. Therefore, in order to remove the resonant frequency of vibrations from the bandwidth of the servo system, bearings of high radial stiffness are required. However, increasing the radial stiffness of the bearings requires a higher bearing pre-load which increases bearing resistance (or rotational friction) and decreases bearing life. As the actuator arm assembly is rotated between the two extreme positions, in-plane vibration can occur. This vibration occurs in the actuator arm assembly substantially in a plane parallel to the plane defined by the magnetic disc.